Convert unit tests to PBT with Hegel

[mrhheffernan]

Description:

Explore property-based testing (PBT) with the Hegel library. Converts existing test coverage over to property-based #[hegel::test] tests.

Key changes:

• Converted to Hegel property tests(generate random inputs via the engine instead of fixed fixtures):
• New property tests added:
  • test_maximin_criterion_square_uniform / test_maxpro_criterion_square_uniform: generate an n×n design whose entries are bounded in [0, 1] and verify the criteria are well-defined.
  • test_calculate_l2_distance_symmetry: asserts dist(a, b) == dist(b, a) and dist(a, a) == 0.

Testing:

• cargo test --lib — all 13 unit + property tests pass.
• New/updated property tests run with cargo test --lib <test_name>.

Impacts:

• Rust
• Python